Effects of agonist and phorbol ester on adrenergic receptors of DDT1 MF-2 cells.

The effects of the adrenergic agonist epinephrine (EPI) and of phorbol 12-myristate 13-acetate (PMA) on the regulation of alpha-1 adrenergic receptors (AAR) and beta adrenergic receptors (BAR) were compared in DDT1 MF-2 cells grown in suspension culture. Pretreatment of cells with 10 microM EPI for 30 min at 37 degrees C resulted in homologous desensitization of BAR-coupled adenylate cyclase activity assayed in membranes and induced internalization or sequestration of BAR. Pretreatment of cells with PMA did not alter BAR-coupled adenylate cyclase activity or induce internalization of BAR. EPI pretreatment caused a 50% decrease in the subsequent ability of EPI to stimulate AAR-mediated incorporation of 32P into phosphatidylinositol, whereas PMA pretreatment inhibited incorporation by 95%. Neither EPI nor PMA induced the internalization of AAR. Neither EPI nor PMA altered agonist binding properties of AAR in short-time competition binding assays on intact cells, indicating that pretreatment of cells with these agents does not alter the affinity of AAR for agonist. In control cells, agonists converted AAR from a form exhibiting predominantly high affinity for agonists, detected in short-time assays, to a form, exhibiting low apparent affinity for agonist during the course of equilibrium competition binding assays. PMA pretreatment increased the extent of this subsequent agonist-induced conversion to the low affinity form. These results indicate that PMA can mimic agonist-induced desensitization of AAR, but not BAR, and that the desensitization of AAR-coupled phosphatidylinositol turnover induced by EPI and by PMA is not due to altered receptor affinity for EPI or due to receptor internalization.